Adjust brightness of LG UltraFine displays

Compiles and works on Ubuntu 16.04 and 18.10

Based on these two projects:

• https://github.com/unknownzerx/lguf-brightness
• https://github.com/csujedihy/LG-Ultrafine-Brightness

Instructions to compile and run

In order to compile this project, you will need g++, cmake, and libusb-1.0-0-dev.

Commands to compile

• cd build
• cmake ..
• make

In order to run this program, you need administrator privileges

sudo ./lguf_brightness 0 100

Compared to the original version, this version ditches curses and takes two command line arguments. The first is the index of the display you want to change the brightness for (0 would be the first, 1 would be the second, etc.) followed by a number between 0 and 100 representing the percent of max brightness to apply. So the above example describes setting the first display to maximum brightness.

Beware, setting brightness to 0 will turn backlighting off.

You can also apply the same brightness settings to all displays by specifying all instead of the display index. So setting all displays to 50% brightness would look like this:

sudo ./lguf_brightness all 50